Prostate cancer is the most common non-cutaneous malignancy diagnosed in American men and is the second leading cause of cancer death in these men. In 1998, an estimated 39,200 men will die as a direct result of prostate cancer. In the absence of a causal agent and given that no method of early detection has been demonstrated to reduce mortality, disease prevention strategies are increasingly important. Soybeans and related legumes contain large quantities of plant estrogens known as isoflavones. Specific isoflavones found in soybeans (genistein, daidzein) have cancer protective properties. Epidemiologic observational studies have suggested that a diet high in soy isoflavones may reduce the incidence and mortality of prostate cancer. Studies in laboratory animals have demonstrated that a diet high in soy can inhibit the development of prostate neoplasms. To date no clinical trials have examined the effects of soy protein supplementation on markers of prostate cancer in humans. This pilot project is designed as a randomized, double-blind, placebo-controlled clinical trial which will assess the effect of a daily soy protein supplement on a number of biomarkers of prostate cancer in men aged 50-70. Prostate biopsies will be taken at baseline and six months following randomization. All tissue will be handled and evaluated in a consistent fashion. The aims of this study are to: 1) identify and recruit men with high PSA levels and normal prostate biopsies, 2) randomize 80 men to receive a daily dietary supplement containing 25 grams of casein protein (control) or 25 grams of soy protein with 58 mg of isoflavones (experimental) for a period of six months 3) assess changes in biomarkers of prostate cancer in prostate tissue obtained before and after 6 months of dietary supplementation, 4) evaluate the effects of dietary supplementation on the participant's health-related quality of life including urinary and sexual function. This project will involve the collaborative efforts of a multidisiciplinary team with expertise in the areas of controlled clinical trials, preventive oncology, nutrition and tissue markers of hormone effects. Evidence for target organ activity with an excellent quality of life profile will provide valuable information for further investigations in the use of soy supplementation to prevent the development of prostate cancer.